Industrial electrolysis, in particular, electrolysis using an inorganic acid as the main electrolyte, has been practiced widely, such as electric metal refining, electric plating, electrolytic syntheses of organic materials and inorganic materials, etc. As the electrode, in particular an anode for these electrolyses, a lead or a lead alloy electrode, a platinum-plated titanium electrode, a carbon electrode, etc., has been used, but these electrodes have shortcomings and, hence, have not been widely used for electrolysis. For example, in a lead electrode, lead dioxide which is relatively stable and has good electrical conductivity is formed on the surface of the electrode but such a lead electrode has the shortcomings that the lead dioxide dissolves at a rather large wear rate of several mg/AH under ordinary electrolytic conditions and also the over-voltage is large. Also, the platinum-plated titanium electrode has a rather short life for high cost. Furthermore, the carbon electrode has the shortcoming that when the anodic reaction is an oxygen generation reaction, the carbon electrode reacts with generated oxygen to consume the electrode itself by forming carbon dioxide, and also the electric conductivity thereof is inferior.
Thus, to overcome these shortcomings of these electrodes, a dimensionally stable electrode (DSE) has been developed and has been widely used.
The DSE is composed of a valve metal such as titanium as the substrate and when the DSE is used as an anode, the surface of the substrate is passivated and the electrode functions as a very chemically stable long-life electrode. However, when the DSE is used as the cathode and is cathodically polarized, the electrode reacts with hydrogen generated at the cathode to form a hydride, whereby the substrate itself becomes brittle and the surface coating is released by corrosion to greatly shorten the life of the electrode and, in particular, a major shortcoming in using the DSE for electrolysis then occurs wherein positive and negative are inverted, that is, the direction of the electric current is reversed.
To avoid the occurrence of these shortcomings, the use of nickel or stainless steel having a durability against the cathodic polarization has been taken into consideration as the substrate material, but since such a material cannot be used as an anode in neutral to acidic solutions, it is clear that the material is unsuitable as an electrode for electrolysis wherein both positive and negative polarizations are given. Also, carbon electrode such as graphite, etc., which is said to be durable to both anodic polarization and cathodic polarization, has a tendency for the surface layer to fall off with generation of gas, and the tendency is particularly large in the electrolysis where both positive and negative polarization occur, whereby the foregoing material may be theoretically usable but the practical usable value is less.
For overcoming the problems in the conventional techniques described above and, in particular, for preventing the occurrence of the passivation of the interlayer, a method of forming the interlayer by flame spraying of tantalum wire onto the substrate is proposed as described in JP-A-5-156480 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). It is reported that in flame spraying of tantalum, an interlayer composed of partial oxide of a mixture of metallic tantalum and tantalum oxide was formed. However; tantalum has the shortcoming that it is easily oxidized; that is, tantalum is more liable to be passivated as compared with other metals; in particular, a long life cannot be expected in the case of using tantalum under severe conditions, and further the use thereof is limited owing to the high cost.
As described above, in the valve metals, iron family metals, and the alloys thereof, materials stable to both the cathodic and anodic polarizations do not exist. However, in ceramics, which are one kind of metal oxides, there exist materials which are stable to both the cathodic and anodic polarizations and give an electric conductivity to a certain extent. However, the electric conductivity of ceramics is far lower than that of metals and further in this kind of electrode being industrially used, a polycrystalline sintered material must be used, whereby such a material has a large electric resistance and is unsuitable as an electrode.